It has been observed that with insulators of this kind, when they are used at relatively high temperatures, e.g. above 60.degree. C., the adhesive or the resin which is used for fixing the end fitting to the end of the shank suffers creep and loses its mechanical characteristics, in particular its traction strength. Consequently, the end fitting can become separated from the shank of the insulator and can be projected at high speed, which is dangerous.
The problem of security in the fixing between an end fitting and an insulator support has already been raised in the prior art, but for insulators for which the support is a solid bar rather than a hollow shank as mentioned above. This has led to end fittings being designed having a bore that is frustoconical in shape, as disclosed in Japanese patent documents Nos. JP 80/36925 and JP 80/36927.